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  • Author or Editor: Susan E. Wijffels x
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Jaclyn N. Brown, J. Stuart Godfrey, and Susan E. Wijffels

Abstract

In a numerical model of the equatorial Pacific Ocean, the ∼20-day period tropical instability waves, excited in the eastern half of the domain, are found to damp the strong zonal mean currents. The waves generate large, nonlinear, advection terms in the momentum balance, change the vorticity balance, and thus modulate the low-frequency state. The authors explore whether the effect of tropical instability waves on the background flow can instead be adequately parameterized by a constant-coefficient Laplacian friction scheme. On annual mean, a Laplacian friction coefficient that varies in space is required, for the coefficient is twice as large along the equator and a few degrees more to the north than elsewhere. In addition, wave activity varies in time. During active phases, such as the second half of the year and during La Niñas, the activity increases, which would require the Laplacian coefficient of friction to be at least twice as strong as during the inactive phases. Thus, a more sophisticated damping parameterization than simple Laplacian friction is required in ocean models that do not explicitly resolve tropical instability waves.

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Susan E. Wijffels, Gary Meyers, and J. Stuart Godfrey

Abstract

Twenty years of monthly or more frequent repeat expendable bathythermograph data are used to estimate the mean geostrophic velocity and transport relative to 750 m of the Indonesian Throughflow (ITF) and its partitioning through the major outflow straits into the Indian Ocean. Ekman transports are estimated from satellite and atmospheric reanalysis wind climatologies. A subsurface maximum near 100 m characterizes the geostrophic ITF, but Ekman flows drive a warm near-surface component as well. A subsurface intensified fresh Makassar Jet feeds the Lombok Strait Throughflow (∼2 Sv; 1Sv ≡ 106 m3 s−1) and an eastward flow along the Nusa Tenggara island chain [the Nusa Tenggara Current (6 Sv)]. This flow feeds a relatively cold 3.0-Sv flow through the Ombai Strait and Savu Sea. About 4–5 Sv pass through Timor Passage, fed by both the Nusa Tenggara Current and likely warmer and saltier flow from the eastern Banda Sea. The Ombai and Timor Throughflow feature distinctly different shear profiles; Ombai has deep-reaching shear with a subsurface velocity maximum near 150 m and so is cold (∼15.5°–17.1°C), while Timor Passage has a surface intensified flow and is warm (∼21.6°–23°C). At the western end of Timor Passage the nascent South Equatorial Current is augmented by recirculation from a strong eastward shallow flow south of the passage. South of the western tip of Java are two mean eastward flows—the very shallow, warm, and fresh South Java Current and a cold salty South Java Undercurrent. These, along with the inflow of the Eastern Gyral Current, recirculate to augment the South Equatorial Current, and greatly increase its salinity compared to that at the outflow passages. The best estimate of the 20-yr-average geostrophic plus Ekman transport is 8.9 ± 1.7 Sv with a transport-weighted temperature of 21.2°C and transport-weighted salinity of 34.73 near 110°E. The warm temperatures of the flow can be reconciled with the much cooler estimates based on mooring data in Makassar Strait by accounting for an unmeasured barotropic and deep component, and local surface heat fluxes that warm the ITF by 2°–4°C during its passage through the region.

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Susan E. Wijffels, Raymond W. Schmitt, Harry L. Bryden, and Anders Stigebrandt

Abstract

The global distribution of freshwater transport in the ocean is presented, based on an integration point at Bering Strait, which connects the Pacific and Atlantic oceans via the Artic Ocean. Through Bering Strait, 0.8 × 106 m3 s−1 of relatively fresh, 32.5 psu, water flows from the Pacific into the Arctic Ocean. Baumgrtner and Reichel's tabulation of the act gain of freshwater by the ocean in 5&deg latitude intervals is then integrated from the reference location at Bering Strait to yield the meridional freshwater transport in each ocean. Freshwater transport in the Pacific is directed northward at nearly all latitudes. In the Atlantic, the freshwater transport is directed southward at all latitudes, with a small southward freshwater transport out of the Atlantic across 35°S. Salt transport, which must be considered jointly with the freshwater transport, is northward throughout the Pacific and southward throughout the Atlantic (in the same direction as the freshwater flux) and is equal to the salt transport through the Bering Strait. The circulation around Australasia associated with the poorly known Pacific-Indian throughflow modifies the above scenario only in the South Pacific and Indian oceans. A moderate choice for the throughflow indicates that it dominates the absolute meridional fluxes of freshwater and salt in these oceans. The global freshwater scheme presented here differs markedly from earlier interpretations and suggests the need for a careful assessment of the treatment of ocean freshwater and salt transports in inverse, numerical, and climate models.

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Bernadette M. Sloyan, Susan E. Wijffels, Bronte Tilbrook, Katsuro Katsumata, Akihiko Murata, and Alison M. Macdonald

Abstract

Repeated occupations of two hydrographic sections in the southwest Pacific basin from the 1990s to 2000s track property changes of Antarctic Bottom Water (AABW). The largest property changes—warming, freshening, increase in total carbon, and decrease in oxygen—are found near the basin’s deep western boundary between 50° and 20°S. The magnitude of the property changes decreases with increasing distance from the western boundary. At the deep western boundary, analysis of the relative importance of AABW (γn > 28.1 kg m−3) freshening, heating, or isopycnal heave suggests that the deep ocean stratification change is the result of both warming and freshening processes. The consistent deep ocean changes near the western boundary of the southwest Pacific basin dispel the notion that the deep ocean is quiescent. High-latitude climate variability is being directly transmitted into the deep southwest Pacific basin and the global deep ocean through dynamic deep western boundary currents.

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